The combustion gases that come out of the tailpipes of our vehicles are comprised of carbon monoxide (CO), hydrocarbons (HC), oxides of nitrogen (NOx), carbon dioxide (CO2), and oxygen (O2). Of these five gases, CO, HC, and NOx are identified as contaminants. These three gases, when combined with heat and sunlight, create what is widely known as smog.

Prior to 1975, leaded gasoline was the only fuel used in internal combustion engines. Starting in 1975, catalytic converters were introduced in new vehicles. All vehicles that were equipped with catalytic converters required the use of unleaded gasoline. The main driver behind the addition of catalytic converters to cars and the introduction of unleaded fuel was the reduction of smog.

TetraEthylLead (TEL) was introduced into gasoline supplies in the 1920s as a means of increasing fuel octane which reduced engine knock and allowed engines to have higher compression ratios. In the late 1940s, a phenomenon was noticed in the air around Los Angeles; the air was turning brown. At first, scientists thought it was a mixture of smoke and fog. So, the term "smog" was coined. After much study, however, smog was determined not to be smoke and fog, but was actually a chemical reaction between sunlight and the exhaust emissions from cars. This was the event horizon for emission control devices on cars.

As a result of the research into smog, pollution control devices were slowly added to new vehicles. In 1961, in an effort to reduce CO and HC emissions, all new model vehicles sold in California were required to have positive crankcase ventilation (PCV) valves installed at the factory. The rest of the country followed suit several years later. In 1973, the exhaust gas recirculation (EGR) valve was mandated as a means to reduce NOx. In 1975, catalytic converters and unleaded fuel started being used in new vehicles, as well as electronic ignition systems. Several years later, electronic feedback carburetors were used to control fuel to the engine more efficiently. Then, in the early 1980s, because feedback carburetors were unable to efficiently control fuel, a replacement system was devised. Electronic fuel injection systems were more adept at controlling fuel into the engine and were more reliable. Since then, computerized engine controls have advanced to the point that virtually every system on every car today is computer controlled.

As a result of all of these technological advances in emission controls, the air quality in our cities has been dramatically improved, even though there are many more vehicles on the road than there were 50 years ago. The air is no longer brown, except on extremely hot, windless days. Respiratory illnesses due to smog are on the decline.

This, in a nutshell, is why it is so vitally important the every car in operation today passes emissions testing that applies to that vehicle.

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